1. Field of the Invention
This invention relates to switching amplifiers and more specifically to such amplifiers in which an improved technique is employed to eliminate the noises which occur when the amplifier is powered up or powered down. The invention is described in the context of an audio frequency amplifier, but the invention also has utility in switching amplifiers operating at other frequencies, or in other applications for which high and low side series connected power transistors such as MOSFETS are used to drive a load from a common node between the transistors.
2. Relevant Art
Switching amplifiers, also commonly known as Class D amplifiers, are characterized by an output stage in the form of a pair of transistors, typically MOSFETS, connected in series between positive and negative sides of a power supply. In the case of audio amplifiers, the common node between the MOSFETS is connected to drive a loudspeaker through a low-pass filter. In operation, the two output transistors function as switches, i.e. they are alternately driven between a substantially fully conductive and a substantially fully non-conductive state. Therefore, except for losses due to Rds of the MOSFET, the voltage at the common output node is alternately switched between the positive and negative supply voltages.
Amplification of the audio signal is achieved by pulse width modulation (PWM) of the gate drive signals for the power transistors, and the amplified is recovered by the low pass filter. To facilitate this, the switching frequency is selected to be very high compared to the audio signal (e.g., 250–300 KHz).
Because the output transistors are either substantially fully on or substantially fully off except during the switching transitions, the class D amplifier exhibits low power consumption and high efficiency. With good circuit design, efficiency of 75% or even as high as 90% can readily be achieved. Moreover, modern Class D amplifiers exhibit excellent audio frequency response and distortion values which are comparable to those of well designed audio amplifiers of other types. Class D amplifiers have been known for almost 50 years, but are finding increasing utility in applications where high heat dissipation (due to high current usage) must be avoided, such as flat panel televisions, and where battery life must be maximized for economy and user convenience, such as in cell phones and other portable audio equipment.
FIG. 1 shows a conventional class D amplifier 10 having a half-bridge topology with two MOSFET output transistors 12 and 14 driving a loudspeaker 16 though an LC filter 18. The audio input signal is provided at 20, and along with a negative feedback signal from a feedback circuit 22, is coupled through an error amplifier 24 to one input of a comparator 26. The other input for comparator 26 is provided by a triangle wave generator 28 to provide a pulse width modulated input signal for a gate drive circuit 30 which controls the operation of MOSFETS 12 and 14.
FIG. 2 shows the output stage of a class D amplifier 40 in a full or H-bridge topology. Here, two MOSFET output transistor pairs 42a–42b and 44a–44b drive a loudspeaker 46 though respective LC filters 48a–48b. This provides added audio output power with the same power supply voltage, and also facilitates open loop operation, but obviously at the price of a more complex and costly circuit.
One of the issues in the design of a class D amplifier is how to deal with the switching noise which occurs during powering up and powering down of the output transistors. Conventionally, this is done by use of a relay between the output circuit and the loudspeaker, but this can add significantly to the size and cost of the amplifier.
An alternative approach which has been considered is to provide a soft start and soft stop by gradually varying the gate drive for the output transistors. For example, it has been proposed to provide a circuit which gradually increases the pulse width of the gate drive signals during a startup interval, and gradually decreases the pulse width during a shut down interval. This is however, not usable in the half bridge topology of FIG. 1 because the inherent DC offset which results from the changing duty cycle causes a clicking noise which is just as objectionable as the switching noise itself.
Another possible approach is to gradually increase the gate voltage for the output MOSFETS by increasing the height of the gate drive pulses, during the start up interval until full switching operation is achieved, and to shut down the amplifier by the reverse process. However, since the on voltage Vth of a MOSFET varies from unit to unit, a voltage imbalance, i.e., DC offset, can still exist, and this must be dealt with in both the half and full bridge topology. A suitable method is therefore still needed to eliminate use of the relay in a class D amplifier to achieve a less expensive and more compact design.